Recovery of chromates by reverse osmosis

ABSTRACT

Process water containing hexavalent chromium is pretreated to bring it to a state of dichromate dominance and the water is then subjected to reverse osmosis using a loose membrane. The membrane allows passage of most of the dichromate ions with the product water while concurrently effecting a significant reduction in the hardness and dissolved solids levels. The softened product water containing dichromate ions is recyclable with the recirculated process water and the wasted fraction contains substantially lower amounts of potentially harmful chromium.

United. States Patent Anthony Geinopolos West Allis;

Mahendra K. Gupta, West Allis; William J. Katz, Milwaukee, all of Wis.

[72] Inventors 21 Appl. No. 143,188 [22] Filed May 13, 1971 [45]Patented Dec. 7, 1971 [73] Assignee Rex Chainbelt Inc.

Milwaukee, Wis.

[54] RECOVERY OF CHROMATES BY REVERSE [56] References Cited UNITEDSTATES PATENTS 3,528,901 9/1970 Wallace et al. 210/23 X PrimaryExaminer-F rank A. Spear, Jr. Anorneys- Ernst W. Schultz and Joseph J.lochman, .I r.

ABSTRACT: Process water containing hexavalent chromium is pretreated tobring it to a state of dichromate dominance and the water is thensubjected to reverse osmosis using a loose membrane. The membrane allowspassage of most of the dichromate ions with the product water whileconcurrently effecting a significant reduction in the hardness anddissolved solids levels. The softened product water containingdichromate ions is recyclable with the recirculated process water andthe wasted fraction contains substantially lower amounts of potentiallyharmful chromium.

BACKGROUND OF THE INVENTION 1 Field of the Invention The presentinvention relates to a process for the selective recovery of chromatesfrom wasted industrial process water and the simultaneous reclamation ofa substantial portion of the wasted process water wherein the chromatesmay be recycled.

2. Description of the Prior Art Industrial operations, such as coolingand condensing in the steel and petroleum industries, require largevolumes of process water. The water which takes on heat in the processis usually cooled in a cooling tower. It is known to treat the processwater with various chemicals to prevent such problems as scaling,corrosion and the growth of algae and bacteria.

It is also know that evaporation in a cooling tower results in anincreasing and eventually undesirable concentration of the dissolvedsolids in the process water which is recycled after cooling. Thus, aportion of the cooled water is continuously purged or bled off. It isreferred to as blowdown" and must be replaced in the system. Make upwater substantially lower in dissolved solids is added to compensate forthe volume lost in evaporation, windage and blowdown and tosimultaneously lower the overall dissolved solids concentration in therecycled process water. Treatment chemicals may also have to be added tomake up for those lost in the blowdown flow. The loss of treatmentchemicals may, in a large cooling tower installation, constitute aconsiderable monetary loss. This factor alone might often make therecovery of one or more of the chemicals for reuse worthwhileeconomically. In addition, recovery and reclamation of the blowdownwater would also provide significant economic benefits.

The blowdown flow is generally discharged to a sewer or otherwisewasted. This flow may, for a 30 F. temperature drop across the tower,constitute 1 percent of the total flow to the cooling tower and canreadily reach a volume of 1,000 gallons per minute. Some of thedissolved solids in the blowdown are potentially hannful and theirdischarge into sewers and waterways has come under strict control.

In particular, the discharge of hexavalent chromium, present in chromatesalts widely used as corrosion inhibitors, has been found to produceparticularly harmful pollution, and controls on its discharge havebecome very stringent. Various methods of recovering chromates fromblowdown and other industrial effluents are known and include ionexchange and electrodialysis. Though these methods have provedsatisfactory for selective removal and recovery of chromates, theprocesses are quite complex and involve multiple recovery andregeneration steps. Further, the usual presence of large amounts ofdissolved solids has made simultaneous reclamation of the water itselfimpractical or impossible.

SUMMARY OF THE INVENTION The present invention involves a process bywhich normally wasted cooling tower blowdown or other process water maybe treated to recover the chromates for reuse and to reclaim most of thewater for recycling with the chromates. It has been found that treatmentof the blowdown containing high concentrations of dichromates and otherdissolved solids by subjecting it to reverse osmosis using a loose-typemembrane effects a significant softening of the water, as well as areduction in the concentration of other dissolved solids, while allowingthe passage of dichromate ions with the product water. This single steptreatment thus provides recovery of chromates, reclamation of water andprevention of chromium pollution of the receiving waters by the wastedportion of the blowdown.

It has been found that chromate salts of monovalent cations are bestsuited to recovery by this process. Prior to the reverse osmosis step,treatment of the blowdown to increase its acidity may be necessary inorder to produce the required dominance of dichromate ions over chromateions.

2 DESCRIPTION OF THE PREFERRED EMBODIMENT There appear to be threespecific prerequisites for the successful recovery of chroma'tes fromprocess water in accordance with the process of this invention, namely:

1. the presence of hexavalent chromium in the form of dichromate anions;

2. the copresence of monvalent cations; and

3. the ability of both of the foregoing ions to pass through a loosereverse osmosis membrane.

The presence of dichromate ions is known to be dependent on the pH ofthe process water. Tests have shown that, if the pH of the solution tobe treated is above approximately 7.0, it must be adjusted downwardly toprovide the condition necessary for dichromate ion dominance. A pH rangeof 4.0 to 6.0 was found to provide the best operation in the process ofthis invention, both to provide high recoveries and to optimize membranelife. The type of acid used to adjust the pH is not believed to becritical and, in actual cooling tower samples tested, the addition ofappropriate amounts of sulfuric acid has provided satisfactory results.

It has been found that, of the inorganic chromate salts commonly used ascorrosion inhibitors in cooling water systems, the chromate salts ofmonovalent cations, such as potassium and sodium, are best suited torecovery by the reverse osmosis process of the invention herein. It wasfound that when the chromate salt of a divalent cation, such as zinc,was utilized as a corrosion inhibitor, the passage of chromate ionsthrough the reverse osmosis membranes was reduced significantly. On theother hand, the recovery of dichromate ions through membrane permeationhas been found to be consistent with the presence and availability ofmonovalent cations.

The loose-type reverse osmosis membrane, found to provide the desiredresults when used in the process of this invention, may be defined as amembrane capable or rejecting not more than percent total dissolvedsolids from a solution containing 5,000 parts per million sodiumchloride. A cellulose acetate membrane of a type well known and widelyused in the art and possessing the required physical characteristics isa Havens Type 310 membrane, which was used in all loose membrance tests,the results of which are set forth below.

The significant characteristic of the loose membrane discovered here isthat it will quite readily allow the passage of dichromate ions, butwill reject substantially all chromate ions. Correspondingly, themonovalent cations of the dichromate salts will readily pass the loosemembrane, as explained above.

In addition, substantial amounts of all other dissolved solids in theprocess water treated are rejected by and will not pass through a loosemembrane. As a result, the product water from the reverse osmosisprocess will be softened significantly and have a dissolved solidscontent consisting principally of dichromate anions and monovalentcations.

Product water recovery of 90 percent of the total treated flow isreadily attainable. This provides a significant reclamation of processwater that is normally wasted, and further provides the ideal medium forcarrying the recovered dichromates back into the cooling water system.Makeup water requirements for the system are, of course, reducedsignificantly.

A tight reverse osmosis membrane may be classified as one capable ofrejecting 96 to 98 percent total dissolved solids (TDS) from a solutioncontaining 5,000 parts per million sodium chloride. Comparative resultsbetween loose and tight membranes are shown in the tables below andthose results show uniformly the unacceptability of a tight membrane inthe disclosed recovery process. In all tests the tight membrane was acellulose acetate Havens Type 510.

The data shown in the tables that follow were obtained in tests using aunit consisting of a bank of series connected modules, each of whichmodules includes a number of tubular membranes connected in series. Thestructure and operation of such a unit are well known in the art. Thefeed water recoveries (product water) obtained in the tests using thisunit ranged from 18 percent to 53 percent depending on membrane type,feed water (influent) flow rate and membrane area utilized. Higher feedwater recoveries are however, readily attainable by simply adding asimilar unit or units in series through which is passed the effluent orflow rejected by the preceding unit.

The feed water recovery generally utilized in actual practice is 90percent. Product water quality at 90 percent feed water recovery wascalculated from the actual data obtained at the lower feed waterrecoveries through the use of a computer program and the calculatedvalues are shown in table ll. The computer program design was based onthe material balance of flow as well as the concentration of ions in theinfluent, product and efiluent streams and it provides an accurateprediction of the product water quality at higher feed water recoveries.

Table I shows the results of a test of the effects of pH and membranetype on the recovery of dichromate in accordance with the teaching ofthis invention. The feed water used in this test was tap watercontaining approximately 30 milligrams per liter potassium dichromateand having an initial pH of 7.6. For the second test run the pH of thefeed water was adjusted to 4.7 by the addition of sulfuric acid. Feedwater at both pH levels was subjected to reverse osmosis with both looseand tight membranes.

Feed water recovery of 53%.

Table ll is based on the results of tests on actual cooling towerblowdown samples. The results indicate that the successful recovery ofhexavalent chromium by reverse osmosis may be made simultaneously withrecovery of substantially all of the blowdown water. The additionalbeneficial softening of the recovered water is shown in the rejection ofhardness and total dissolved solids (TDS). As shown in the table, oneblowdown sample contained potassium dichromate as a corrosion inhibitorand the other sample contained zinc dichromate for the same purpose. ThepH of both samples was adjusted to 5.5

and each was subjected to treatment through both a loose and a tightmembrane.

Calculated from actual feed water recoveries of l8% to 53%.

The table ll results indicate a 77 percent dichromate recovery atpercent feed water recovery in accordance with the process of thepreferred embodiment. These are considered to be practicable recoveriesfrom an economic standpoint and significant recoveries from thestandpoint of pollution abatement and water reclamation.

What IS claimed is:

l. A method for treating process water having a high dissolved solidscontent to make the water recyclable and to retain for reuse in therecyclable fraction a substantial portion of the chromium ions whichcomprises:

a. utilizing a chromate salt of a monovalent cation as the chromiumsource in the process water;

b. adjusting the pH of the water to be treated to less than 7.0; and

c. subjecting the water to reverse osmosis utilizing a membrane capableof rejecting tot more than 90 percent total dissolved solids from asolution containing 5,000 parts per million sodium chloride.

2. A method for recovering the dichromate salt of a monovalent cationfrom an aqueous solution having a high concentration of dissolved solidscomprising the steps of:

a. adjusting the pH of the solution to less than 7.0;

b. subjecting the solution to reverse osmosis utilizing a membranecapable of rejecting not more than 90 percent total dissolved solidsfrom a solution containing 5,000 parts per million sodium chloride; and

c. retaining the product water from the preceding step.

3. The method in accordance with claim 2 wherein the pH of the solutionto be treated is adjusted to a value of from 4.0 to 6.0.

i I t t

2. A method for recovering the dichromate salt of a monovalent cation from an aqueous solution having a high concentration of dissolved solids comprising the steps of: a. adjusting the pH of the solution to less than 7.0; b. subjecting the solution to reverse osmosis utilizing a membrane capable of rejecting not more than 90 percent total dissolved solids from a solution containing 5,000 parts per million sodium chloride; and c. retaining the product water from the preceding step.
 3. The method in accordance with claim 2 wherein the pH of the solution to be treated is adjusted to a value of from 4.0 to 6.0. 